Missing version combinations in continuous integration

[yhoiseth]

Thanks a lot for your work on this very useful library.

According to .travis.yml, a lot of version combinations are tested:

env:
    - TOX_ENV=py27-flake8
    - TOX_ENV=py27-docs
    - TOX_ENV=py27-django1.8-drf3.5
    - TOX_ENV=py27-django1.8-drf3.6
    - TOX_ENV=py27-django1.10-drf3.5
    - TOX_ENV=py27-django1.10-drf3.6
    - TOX_ENV=py34-django1.8-drf3.5
    - TOX_ENV=py34-django1.8-drf3.6
    - TOX_ENV=py34-django1.10-drf3.5
    - TOX_ENV=py34-django1.10-drf3.6
    - TOX_ENV=py35-django1.8-drf3.5
    - TOX_ENV=py35-django1.8-drf3.6
    - TOX_ENV=py35-django1.10-drf3.5
    - TOX_ENV=py35-django1.10-drf3.6
    - TOX_ENV=py36-django1.8-drf3.5
    - TOX_ENV=py36-django1.8-drf3.6
    - TOX_ENV=py36-django1.10-drf3.5
    - TOX_ENV=py36-django1.10-drf3.6

However, there are a few (more recent) combinations that are not tested. For example, we're running Python 3.7, Django 2.1 and Django Rest Framework 3.9.

By adding more combinations, we could potentially discover problems that may occur on those.

I'd be happy to contribute by adding more version combinations.

[yhoiseth]

What are the versions required by Django and DRF?

Django has the following requirements:

Django version	Python versions
1.11	2.7, 3.4, 3.5, 3.6, 3.7 (added in 1.11.17)
2.0	3.4, 3.5, 3.6, 3.7
2.1, 2.2	3.5, 3.6, 3.7

Django Rest Framework (3.9, I assume) requires:

Python (2.7, 3.4, 3.5, 3.6, 3.7)
Django (1.11, 2.0, 2.1, 2.2)

[johnthagen]

FYI, Python 3.4 is EOL, so you'll notice the latest DRF (3.9 at the moment) doesn't support it.

[johnthagen]

@dbkaplan the last commit in this repository was over 1 year ago. Is this package still maintained? (#60)